In a typical digital camera, a lens projects an image of a scene onto an electronic array light sensor. The sensor and associated electronics read the impinging light, and construct an array of numerical values representing the brightness, color, or both of scene locations. This array of numerical values may be called a digital photograph, a digital image, or simply an image or photograph.
While a camera user is composing a photograph, the camera may take many preliminary photographs in preparation for taking a “final” photograph. For the purposes of this disclosure, a “final” photograph is the photograph the photographer intends to take and store for later use. At least some of the preliminary photographs may be used for gathering information about the scene so that the camera may be configured properly for taking the final photograph. For example, some of the preliminary photographs may be analyzed to determine the brightness of the scene so that exposure parameters, such as a lens aperture and exposure time, may be set for taking the final photograph. The distribution of colors in some of the preliminary photographs may be examined to determine what kind of white balance adjustment should be applied to the final photograph.
Some of the preliminary photographs may be displayed on an electronic display on the camera so that the user can see an approximation of what the final photograph may look like. When sequential preliminary photographs are shown on the display, the resulting “live view” aids the photographer in composing the final photograph. In some digital cameras, this live view is the only means the photographer has for composing a photograph, as some digital cameras do not provide an optical viewfinder.
Some of the preliminary photographs may be used for focusing the camera's lens. Generally, focus is adjusted by moving one or more lens elements, and the quality of focus at a particular focus setting is evaluated by computing a spatial contrast metric for a photograph taken at that focus setting. Focusing may be accomplished by computing the spatial contrast metric for photographs taken at more than one trial focus settings, and then adjusting the focus position based on the computed metrics until focus is optimized. The process may be iterative and may require the taking of several trial photographs. Some cameras, for example digital single lens reflex (SLR) cameras, may have a separate sensor dedicated to focusing so that the main electronic array light sensor can be used for taking live view preliminary photographs without interruption during focusing. However, this additional sensor adds cost and complexity to the camera. It is desirable to use a single electronic array light sensor for both live view and for focusing, and for other functions the camera performs.
These many different uses for preliminary photographs have divergent requirements. It is desirable that photographs shown on the display during live view be of good quality, with little noise and accurate colors. This may require that a photograph taken for live view have a relatively long exposure time, especially when the scene is dimly lit. However, photographs used for automatic focusing should have relatively short exposure times so that subject motion has minimal effect on the spatial contrast analysis, and so that focusing can be accomplished rapidly.
Because these requirements conflict, previous cameras have simply suspended the live view during automatic focusing. This may be disconcerting for a photographer who depends on the live view for composition. Furthermore, because continuous focusing and live view have been incompatible, many consumer digital cameras are inconvenient for photographing rapidly changing scenes such as sporting events.